Communication devices such as wireless communication devices, that simply may be named wireless devices, may also be known as e.g. user equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. A wireless device is enabled to communicate wirelessly in a cellular communication network, wireless communication system, or radio communication system, sometimes also referred to as a cellular radio system, cellular network or cellular communication system. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communication network. The wireless device may further be referred to as a mobile telephone, cellular telephone, laptop, Personal Digital Assistant (PDA), tablet computer, just to mention some further examples. Wireless devices may be so called Machine to Machine (M2M) devices or Machine Type of Communication (MTC) devices, i.e. devices that are not associated with a conventional user.
The wireless device may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile device, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The cellular communication network covers a geographical area which is divided into cell areas, wherein each cell area is served by at least one base station, or Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is typically identified by one or more cell identities. The base station at a base station site provides radio coverage for one or more cells. A cell is thus associated with a geographical area where radio coverage for that cell is provided by the base station at the base station site. Cells may overlap so that several cells cover the same geographical area. By the base station providing or serving a cell is meant that the base station provides radio coverage such that one or more wireless devices located in the geographical area where the radio coverage is provided may be served by the base station in said cell. When a wireless device is said to be served in or by a cell this implies that the wireless device is served by the base station providing radio coverage for the cell. One base station may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless device within range of the base stations.
In some RANs, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunication System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communication (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to other base stations and may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which may be referred to as 3rd generation or 3G, and which evolved from the GSM, and provides improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for wireless devices. High Speed Packet Access (HSPA) is an amalgamation of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), defined by 3GPP, that extends and improves the performance of existing 3rd generation mobile telecommunication networks utilizing the WCDMA. Such networks may be named WCDMA/HSPA.
The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example into evolved UTRAN (E-UTRAN) used in LTE.
The expression downlink (DL) is used for the transmission path from the base station to the wireless device. The expression uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
Existing radio access technologies deploy different multiplexing/multiple access schemes to divide the radio spectrum among multiple users. For example in GSM, TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access) are used, while TDMA and OFDMA (Orthogonal Frequency Division Multiple Access) are used in LTE.
When the multiplexing scheme has a TDMA component, the transmitted signal is divided in blocks, transmitted sequentially in time on e.g. the same radio frequency channel. Multiplexing is provided by alternating between blocks to different receivers.
On most radio channels, the transmitted signal is distorted by the channel propagation before it reaches the receiver. The channel may e.g. add time dispersion and attenuation. Typically, the impact varies with time due to e.g. movement of the transmitter and/or receiver. In addition, the transmitter and receiver may themselves introduce random changes in the phase of the signal from one block to the next.
In some situations, e.g. when the transmitter and receiver are stationary, the channel variations are very slow or even non-existent. In such scenarios, a well-known technique to increase the coverage is to use block repetition on the transmitter side and coherent accumulation of multiple received signal samples on the receiver side. Block repetition means that the same information is being sent multiple times, i.e. repeatedly. A pre-requisite for coherent combining is also that the phase of the transmitted blocks is the same or changes in a known way.
With block repetition, the SNR (signal-to-noise ratio) after coherent combining will increase as 10*log10(N), where N is the number of repetitions. This can be understood as follows:
The bursts of the wanted signal will be added coherently, meaning that the amplitudes of the signal samples will be added. If N bursts are added, the amplitude will increase N times, and consequently, the energy of the wanted signal will increase by a factor N2.
The noise samples of each burst will also be added, but assuming that the noise samples are independent, they will not be added coherently. Addition of independent variables means that the variance of the sum equals the sum of the individual variances. Therefore the variance, and hence the energy, of the noise will increase by a factor N.
Consequently, the SNR will increase by a factor N2/N=N, or 10*log10(N) if expressed in dB.
Block repetition has been proposed for example in LTE and GSM in order to achieve coverage increases of up to 15-20 dB for Machine Type Communications (MTC), see e.g. 3GPP TR 36.888 v12.0.0 “Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE”, and 3GPP TR45.820 v1.0.0 (GP-150317).
While block repetition increases the effective SNR in a noise-limited scenario, it will not increase the effective SIR (signal-to-interference ratio) to the same extent in case the link is interfered by an interferer that also uses coherent burst repetitions, since the interference samples will also add up coherently. Therefore, the interference energy will increase by a factor N2, just as the wanted signal, and the carrier-to-interference ratio will not increase by the use of burst repetitions. If the interferer uses a different repetition interval, and/or has a time offset relative to the wanted signal, the interferer will in part be added coherently and in part non-coherently, and the interferer energy amplification will be somewhere in between N and N2. In general, the gain of repetitions will be less when subject to interference than to noise.